A LAN is a high-speed network that supports many computers connected over a limited distance (e.g., under a few hundred meters). A Virtual Local Area Network (VLAN) is mechanism by which a group of devices on one or more LANs is configured using management software so that they can communicate as if they were attached to the same LAN, when in fact they are located on a number of different LAN segments. Since VLANs commonly span many switches across different LAN segments, sharing of Virtual LANs by a common set of infrastructure switches is achieved by inserting a VLAN identifier (VID) or tag into the Ethernet frame header to provide differentiation between traffic flow, i.e., separate service or customer instance. The customer identifier is frequently referred to as the service instance identifier since it identifies the service provided for a particular customer. A Virtual Private LAN Service (VPLS) service emulates a VLAN over an MPLS/IP network allowing the sites for a given VLAN to be geographically dispersed. If these sites are located in different Administrative System domains (ASes), then Multi-Protocol Border Gateway Protocol (MP-BGP) is used for communication across these domains for an MPLS/IP network.
Currently, bridged services for Metro Ethernet networks (ELAN or EVLAN) are offered over MPLS using an overlay topology where Provider Edge devices (PEs) are connected using pseudowires (PWs). A PW is a virtual connection between two PE devices. In the context of the VPLS service, a PW can be thought of as point-to-point virtual link for each service offered between a pair of Virtual Switch Instances (VSIs) within the PEs that emulates an Ethernet Virtual LAN function in terms of media access control (MAC) address learning and forwarding. Each VSI can be thought of as a virtual Ethernet switch for a given customer service instance, and each PW can be thought of as a virtual link connecting these virtual switches over a Packet Switched Network.